1. Field of the Invention
The present invention relates to a recording sheet to be used for recording information thereon using ink or the like. More specifically, the invention relates to a recording sheet suitable for printing information thereon by an ink jet printer when pigment ink is used.
2. Description of the Related Art
Prior recording systems have been adapted to output printers of computers, word processors, and so on. Typically, such recording systems include a wire dot recording system, a thermal coloring recording system, a thermal melting transfer recording system, a thermal sublimation transfer recording system an electrophotographic recording system, and an ink jet recording system.
Of the types listed above, the ink jet recording system has excellent features of: the capability of using a wood free paper as a recording sheet; a low printing cost; a low noise during the printing movement; a small-sized printing device; a high speed printing; and so on, compared with those of the other recording systems. In recent years, therefore, there is the increasing range of uses for the ink jet recording system.
Various kinds of the recording sheets to be used in such an ink jet recording system are proposed in the art. In FIG. 6(a), reference numeral 110 denotes an example of the recording sheet to be used in a typical ink jet recording system.
The recording sheet 110 includes a transparent substrate 111, an ink receptive layer 112 formed on the surface of the substrate 111, and an ink permeable layer 113 formed on the surface of the ink receptive layer 112.
In the case of performing an ink jet recording movement on the recording sheet 110, ink is discharged as a plurality of ink droplets 114 from nozzles of an ink jet printer to the surface of the ink permeable layer 113 (FIG. 6(a)).
The ink permeable layer 113 of the recording sheet 110 generally includes additives such as organic and inorganic fillers. If the filler particles are dispersed in the resin provided as the binder of the ink permeable layer 113, the gaps between adjacent filler particles allow the formation of a porous structure in the ink permeable layer 113.
Once the ink droplets 114 touch on the surface of the ink permeable layer 113, the ink droplets 114 permeate from the surface to the inside by passing through “holes” in the ink permeable layer 113, (i.e., the ink permeable layer 113 is porous).
The ink droplets 114 continue to permeate through the ink permeable layer 113 until reaching the ink receptive layer 112. Once the ink droplets 114 reach the ink receptive layer 112, the ink droplets 114 are absorbed by the ink receptive layer 112, forming a dot 117 of ink thereon.
The ink droplets 114 absorbed in the ink receptive layer 112 can be observed as their corresponding dots 117 from the other side of the transparent substrate 111 with respect to the ink receptive layer 112. Therefore, a set of these dots 117 can be observed as an image printed on the recording sheet 110 (FIG. 6(b)).
In recent years, such recording sheets 110 have been extensively used for overhead projectors and in many other applications.
Furthermore, as described in Japanese Patent Laid-Open Publication No. Sho. 62-280068, if a surfactant is added to the ink permeable layer 113, it is possible to increase the ink permeability of the ink permeable layer 113 with respect to ink that uses a dye (“the dye ink”) as a colorant.
However, if pigment ink is used instead of the dye ink, a similar effect does not occur. In the pigment ink, pigments exist as dispersed particles in the ink. In the dye ink, on the other hand, the dye is completely dissolved in the ink. Therefore, the pigments provided as colorant particles have difficulty permeating through the ink permeable layer 113 described above, so that the pigments can accumulate in the ink permeable layer 113.
As a result, the absorbed content of colored component in the ink receptive layer 112 becomes low, so that the printing density of the image (reflective image) observed from the surface of the substrate 110 becomes low.
In addition, the recording sheet 110 described in the Japanese Patent Laid-Open Publication No. Sho. 62-280068 includes a hydrophobic organic filler in the ink permeable layer 113. Such an organic filler is commonly expensive in comparison with inorganic one, so that the cost of the recording sheet 110 rises as a whole.
Another prior art system uses low-priced silica instead of a hydrophobic organic filler. The surface of the silica is covered with hydrophilic groups (which are similar to silanol groups), so that the silica has a high affinity for water-based ink. Therefore, if silica is added to the ink permeable layer 113, the water-based ink penetrates not only vertically (depth direction) but also laterally in the ink permeable layer 113. As a result, the ink is diffused broadly in the ink permeable layer 113. When the ink is diffused broadly, different ink droplets 114 may be mixed together in the ink permeable layer 113. As a result, neighboring dots 117 of a printed image are overlapped. Thus, the overlapped portion can be observed as a smear (banding) on a printed image.